Resolving intra-atomic electron dynamics with attosecond transient absorption spectroscopy

Attosecond transient absorption spectroscopy is a recent addition to the experimental tool set of attosecond science. This all-optical method measures different observables than the previously existing techniques based on electron and ion detection and overcomes several of their limitations. We review the present state-of-the-art of attosecond transient absorption experiments and theory. Applications cover the exploration of ultrafast electron dynamics in atoms as well as in solid-state systems. As an example we discuss the observation of transiently bound electron wavepacket dynamics in helium in more detail. This example illustrates how transient absorption spectroscopy can provide information that is fundamentally inaccessible to the techniques based on ionisation, namely dynamics occurring well below the ionisation threshold. Furthermore, we show that a model based on wavepacket interference and originally developed to explain modulations in the ion yield is not sufficient to explain the observed optical response of the system. The optical response on extremely short timescales and in systems exposed to strong laser fields is still not fully understood. This makes the method also attractive for fundamental studies. Furthermore, it is expected that the technique will also find future applications for studying molecular systems in gas phase, in solution, or as solids and will greatly benefit from the advances of ultrafast lasers with multi-100-W average power improving signal-to-noise ratio by many orders of magnitude in the near future.